This invention relates to ophthalmic instruments and, in particular, to such an instrument utilized in qualitatively determining an amount of astigmatism associated with a cornea of a human eye.
During certain surgery on the eye, for example a cataract operation, it is necessary to make an incision in the periphery of the cornea. After the operation, the incision must be sutured, which often creates a certain degree of astigmatism. Preferably, the the cornea should be reconfigured to its pre-operative shape or a shape having as little astigmatism as possible, so that any astigmatism associated therewith will not need correction or alternatively can be corrected fairly easily with corrective lenses. Perhaps the most well-known device that is used to ascertain measurements such as corneal astigmatism or curvature is the ophthalmometer. For example, Bausch and Lomb Corporation markets an ophthalmometer under the trademark "Keratometer". The ophthalmometer measures the anterior curvature of the cornea by interpretation of the measured size of a corneal image created by an object of known size located a fixed distance from the cornea. A ray of light is reflected onto the cornea and the resulting elliptical image is analyzed through a series of prisms. The typical ophthalmometer is a sophisticated, bulky, and extremely expensive piece of equipment. The device is not easily used in an operating room, due mainly to its bulk and the amount of time required to manipulate the device before quantitatively determining the corneal curvature.
Various other devices have been made to quantitatively determine the corneal astigmatism, or ellipticity associated with the cornea. These devices tend to be bulky and difficult to use in an operating room or tend to provide only extremely rough approximations. Some such devices are similar to ophthalmometers in that they use prisms to focus the reflected image in order to quantify the ellipticity. Certain measurement processes require extensive computer equipment to properly analyze the data developed thereby.
Another disadvantage that most of the complex prior art instruments have is that they require extensive manipulation to obtain the quantitative determination. Such manipulation is time consuming and tedious as the equipment is both bulky and somewhat fragile. These instruments are not easily sterilized and present a potential threat of contamination in the operating room.
Certain ophthalmic instruments rely on qualitative methods, but they are generally not accurate enough to provide significant information with respect to the amount of corneal astigmatism. Other devices have been too imprecise in giving comparative information and only provide an image of light shown through a circular opening. One such device that has been utilized is the circular spring portion of a safety pin.
Yet another disadvantage associated with certain of the known apparatus is that they require the degree of astigmatism to be estimated indirectly. That is, they do not allow the degree of astigmatism to be determined as an end point, but rather require the degree to be estimated based on the amount of ellipticity associated with the observed cornea.